MZT1 regulates microtubule nucleation by linking γTuRC assembly to adapter-mediated targeting and activation

Regulation of the gamma-tubulin ring complex (gamma TuRC) through targeting and activation restricts nucleation of microtubules to microtubuleorganizing centers (MTOCs), aiding in the assembly of ordered microtubule arrays. However, the mechanistic basis of this important regulation remains poorly understood. Here, we show that, in human cells,gamma TuRC integrity, determined by the presence of gamma-tubulin complex proteins (GCPs; also known as TUBGCPs) 2-6, is a prerequisite for interaction with the targeting factor NEDD1, impacting on essentially all gamma-tubulin-dependent functions. Recognition of gamma TuRC integrity is mediated by MZT1, which binds not only to the GCP3 subunit as previously shown, but cooperatively also to other GCPs through a conserved hydrophobic motif present in the N-termini of GCP2, GCP3, GCP5 and GCP6. MZT1 knockdown causes severe cellular defects under conditions that leave gamma TuRC intact, suggesting that the essential function of MZT1 is not in gamma TuRC assembly. Instead, MZT1 specifically binds fully assembled gamma TuRC to enable interaction with NEDD1 for targeting, and with the CM1 domain of CDK5RAP2 for stimulating nucleation activity. Thus, MZT1 is a 'priming factor' for gamma TuRC that allows spatial regulation of nucleation.